How large is the bonding space? Sometimes a very large space can be helpful (it helped with mine!)
And did you do any pre-bonding? (usually 1 month is recommended, this means cage swaps with no actual interaction)
Also, how long has each been spayed/neutered? (boys can take 3-6 weeks for hormones to drain, girls can take several months)
For stressing, car-rides are my go-to.
With bunnies, the bum is a very sensitive area, so her sniffing his but may not be as innocent as it seems. Could indicate she wants to mount, or as asking for grooms, but at the wrong end. :p
Depending on your answers to my questions, my suggestions will vary. But, assuming you have done pre-bonding and both have their hormones settled, I would recommend trying a very large, new, very neutral space. Especially since your boy is a giant, give them lots of space. Like, at least 2 x-pen’s worth, maybe more. You can add in tunnels and 3 or 4 hiding boxes (with at least two exits). Using a friend’s backyard, kitchen, or garage would work well. The rescues in your area might even allow you to bring them in and use their exercise pens.
When bunnies are in small spaces, things can escalate to a fight very quickly, as they have no where to escape to. I am fairly sure at this point that when bunnies are aggressive, it is almost always due to fear. If you give them space to escape, they are less likely to escalate to a full fight.
The other technique that has worked very well for me, especially in beginning stages like yours, is to pet them both A LOT whenever they approach. By now they have likely gotten into a pattern of “The other rabbit is approaching me, I don’t like them”. So next time they approach each other, start petting them both so much that you flatten them into the ground and keep them calm. You can also start a date like this, with just petting them side by side. You want to build a calm association and trust between them.
If you plan to keep working with them at home, since you say they fight after 30 seconds, end the date at 29 seconds so it doesn’t end on a fight. Stick with short sessions where they don’t fight, then slowly increase the time they spend together…
Reading bonding journals lets you see the full process and how long things can really take (and how often people almost quit!). There are often lots of ups and downs!
Here are some that might help you:
Vienna’s bonding thread (she bonded her two HUGE beautiful bunnies after having a very rough go of it at first): https://binkybunny.com/FORUM/tabid/54/aft/150603/Default.aspx
And here’s my most recent bonding process, where I used the tips I gave you:
https://binkybunny.com/FORUM/tabid/54/aft/164900/Default.aspx
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.